Step 1: Take a blank letter/A4 paper

Step 2: First Fold

Fold upper right corner to left side, making a 45 degree right triangle above the rest of the paper.

Step 3: Second Fold

Mountain Fold (away, behind) the paper vertically in half, so the right half is behind the left half.

Step 4: Loop Fold up

The next step is two folds:

Fold the bottom part up along the line created by the bottom of the triangle/trapezoid.
Fold the bottom up again, the same size as the previous fold.

Step 5: Almost there...

Fold the triangle down over the rectangle (Picture is rotated 90 degrees).

Step 6: Tuck and go!

This one is a bit tricky -- It's an optional Z fold.
The idea is to push the triangle into the crease, and here's your choice: Either leave a bit of the triangle tab out, or tuck it all the way inside. If you leave the triangle tab out, try to keep the tip below the outer rectangle fold, and have the triangle tab lined up with the other triangle in front.

Your comment doesn't explain enough why this won't work for A4. Because I don't have A4 paper, I tried with 7.777x11 paper (the aspect ratio is equivalent) and obtained a squarish result, but still with a small interlocking tab. To obtain results similar to the above for A4 paper, after the initial triangle fold, make a 25mm (1 inch) valley fold from the bottom rectangle/remainder edge up. This would adjust the proportions to resemble the folds above.

Girls at my school just text on their phones, it's kind of annoying. The guys yell whatever they want to say across the room (it ticks off the teachers to the extreme). I'm gonna use this though, so very nice instructable.

They were common for me at my school, too, 20 years ago. The problem for me is that I almost forgot how to do it, for whatever reason I needed to do it now, and once I remembered, I thought I'd place something to remind myself how to do it. Thank you kindly for your comment.